The conventional front wheel suspension has a turning circle which is defined as the smallest turning radius that the front wheels may turn with the steering wheel angling the vehicle wheels to the maximum extent. The radius is between a rotational center located on line extended through the rear wheel axis to a wheel pivot axis at a stub axle about which the wheel pivots. The maximum angle through which the front wheels can turn is usually called the locking angle. There is a front locking angle for the near front wheel and a rear locking angle for the farther front wheel. One limitation on the extent of front wheel pivoting is the abutment of a caliper of a front wheel disc brake with another portion of the suspension such as a top or bottom support member for the stub axle. The top and bottom supports are often wishbone suspension members or a McPherson strut suspension member. The top and bottom support members support the stub axle and are connected by ball connections to the stub axle for turning it about an axis through these ball joint connections. A steering suspension member is connected at one end to a rack and pinion steering device, or the like; and it is pivotally connected at its other end to the stub axle. The stub axle also carries a hub for mounting the wheel for rotation.
The conventional caliper disc brake is usually mounted at a rear location in the sense of its being behind the wheel axle as the vehicle is traveling in a forward direction. This rearward location of the caliper rearwardly results in protection of the braking mechanisms and steering arm from being damaged during a crash because the mass of the motor protects them if the front end of the vehicle is being crushed into the motor during an accident. Although it is possible to locate the caliper forwardly on the brake disc, rather than rearwardly, then the caliper is not so protected by the vehicle engine during a crash. Moreover, the wheel is turning downwardly at a caliper front location, and the weight of the vehicle is also directed downwardly onto the stub axle; and these downward forces are additive during a braking operation when the caliper is located forwardly. Because the wheel rotation is up at a rear caliper location, an upward force from the wheel opposes the downward force due to the weight of the vehicle on the stub axle. Because of the additive effects due to a front caliper positioning, the stub axle and wheel bearings often are made larger to provide increased stiffness and load carrying capabilities that may not be needed when the caliper is rearwardly mounted.
The relatively heavy weight of a conventional stub axle and its large sliding caliper are undesirable because the stub axle and wheel are an unsprung mass. Even for small automobiles, the combined weight of the stub axle and conventional caliper brake mechanism is about 18 kg. Automobile manufacturers are trying to decrease this unsprung weight of this portion of the front wheel suspension to improve driving characteristics and lower fuel consumption.
The conventional caliper disc brake is not mounted at the top portion of the stub axle for several reasons, one of which is that of "knock back" of the hydraulic piston in the hydraulic cylinder of the floating, slidable caliper. More specifically, during cornering which develops high side loads, the brake disc fixed to the wheel hub pushes on the brake pad and pad carrier and thereby pushes the piston back into the cylinder by several thousandths of an inch or more resulting in a subsequent longer piston stroke than usual for braking the vehicle to a stop. This "fall back" of the piston deeper within the cylinder requires a longer travel of the piston when the brakes are next applied, and results in a long pedal feel to the vehicle driver. Fall back is avoided by putting the caliper at a front or rear location.
The conventional caliper disc brake includes a slide bolted to the stub axle with the brake cylinder and piston within this slide. At the onset of brake application, the hydraulic fluid in the cylinder shifts the piston, and it moves a brake carrier and brake pad thereon to abut one side of the fixed brake disc; and through a reaction force, the caliper slide shifts to bring a fixed brake pad on a distal end of the slide into engagement with the opposite face of the fixed brake disc. In order to contain the hydraulic cylinder and piston therein and to provide the requisite stiffness for the support of the fixed brake pad during a braking operation, the caliper slide or housing is usually quite large and heavy. Again, this weight is an unsprung weight, and hence, undesirable. Further, the large size of the caliper increases its outward projection and hence, its ability to foul with a suspension member. The exact fouling location is difficult to describe precisely because when traveling over rough roads, the suspension member shifts vertically and can abut the fully turned caliper brake at locations it may not abut when traveling over a smooth road.
The conventional caliper extends a considerable distance radially outwardly a considerable distance from the wheel vertical turning axis. Hence, it is desirable to have a brake assembly that does not project radially outwardly so far from the vertical turning axis to reduce fouling of the brake mechanism with a suspension member.
Although the caliper slide or housing are usually quite large, its distal end of a bridge carrying the fixed brake pad is still subjected to deflection to an undesirable extent at the forces exerted by a line pressure, which is usually 70 BARS or more for a high coefficient of friction road surface. This deflection causes hysteresis loss when the brake apply and release cycles are very frequent, as during an ABS controlled iteration of wheel deceleration and wheel acceleration cycles. Further, the deflection may cause a problem with the return of the piston to the exact location in the cylinder in the cylinder. The return of the piston is by a return force from a seal compressed during outward travel of the piston; and this force and travel are limited, and unwanted deflection of the distal slide end may cause problems with the proper positioning of the piston in the cylinder. Many efforts have been made to increase the stiffness of the caliper slide to limit this deflection of the distal end of the slide caliper without increasing too much the size or the weight of the caliper slide.
In addition to being heavy and large, the conventional caliper, disc brake system has a relatively large, residual brake torque, which is the braking torque experienced due to a rubbing between the brake pads and fixed brake disc while the brake system is in an off-brake condition. Residual torque wastes fuel, decreases the life of the brake pads and disc, and increases the initial temperature of the brake pads and discs at the start of fade tests, such as the AMS fade test. Moreover, the residual brake torque retards the vehicle wheel during a wheel speed up portion of an ABS iterative wheel acceleration/deceleration cycle. This results in a slowing down the iterative cycle during an ABS braking operation.
The present invention is directed to a slidable, brake disc assembly mounted on a stub axle and suspension to reduce the turning circle by providing a more compact brake disc and stub axle assembly that does not foul in the manner of a conventional, slidable caliper brake having a fixed brake disc. There exists a fair number of slidable disc brake patents which disclose the broad concept of slidable brake discs and some use a large caliper, which is attached to the stub axle and the caliper projects radially outwardly from the stub axle by a considerable extent to provide a large radius from the wheel turning axis to the outer end of the caliper. It is desired to provide a more compact and low profile, slidable brake disc and stub axle assembly than disclosed in these patents. Many of these patents disclose slidable brake disc assemblies without disclosing a stub axle or knuckle; hence, they are unconcerned with providing a brake and knuckle assembly that has a low profile and is compact to improve the turning circle of the vehicle. Moreover, these patents disclose slidable, disc brake systems that are not currently used in original equipment vehicles because they have shortcomings in their operation, which have been overcome in this invention.
From the foregoing, it will be seen that there is a need for an improved wheel suspension having a slidable disc brake and stub axle assembly that does not limit the turning circle to the extent limited by the conventional caliper brakes and which is lighter in weight and has better operational characteristics.